A Northwestern University research team is the first to demonstrate that a material called metal-organic framework (MOF) is a stable and selective catalyst for breaking down polyester-based plastic into its component parts.

It helps a used plastic bottle to be returned to its original components, ready to be made into a new plastic bottle instead of possibly ending up in a landfill. The work was published recently in the journal Angewandte Chemie.

Only three things are needed: plastic, hydrogen and the catalyst. An important bonus is that one of the component parts the plastic is broken down into is terephthalic acid, a chemical used to produce plastic. With the Northwestern method, it isn’t necessary to go all the way back to oil and the expensive and energy-intensive production and separation of xylenes.

“We can do a lot better than starting from scratch when making plastic bottles,” shared Omar Farha, a professor of chemistry in the Weinberg College of Arts and Sciences who is the corresponding author of the study. “Our process is much cleaner.”

The researchers chose a zirconium-based MOF called UiO-66 because it is easy to make, scalable and inexpensive. Yufang Wu, the study’s first author and a visiting graduate student in Omar Farha’s group, used the plastic that was most handy - the plastic water bottles her colleagues in the lab had discarded. She chopped them up, heated the plastic and applied the catalyst.

Catalytic degradation of PET using a phase-transitional zirconium-based MOF.

“The MOF performed even better than we anticipated,” Omar Farha told. “We found the catalyst to be very selective and robust. Neither the color of the plastic bottle or the different plastic the bottle caps were made from affected the efficiency of the catalyst. And the method doesn’t require organic solvents, which is a plus.”

A class of nano-sized materials, MOFs have been widely investigated because of their highly ordered structures. Omar Farha has studied MOFs for more than a decade and previously showed they can be used to destroy toxic nerve agents.

In the current study, MOFs act also in much the same way — breaking an ester bond to degrade polyethylene terephthalate (PET).

“We’ve been using zirconium MOFs to degrade nerve agents for years,” Omar Farha said. “The team then wondered if these MOFs could also degrade plastic even though the reactions and mechanism are different. That curiosity led to our recent findings.”

“This research helps to address long-standing challenges associated with plastic waste and opens up new areas and applications for MOFs,” Farha continued.

MOFs are made of organic molecules and metal ions or clusters which self-assemble to form multidimensional, highly crystalline, porous frameworks. To picture the structure of a MOF, Omar Farha explained, envision a set of Tinkertoys in which the metal ions or clusters are the circular or square nodes and the organic molecules are the rods holding the nodes together.

In addition to being easy to make, scalable and inexpensive, another advantage of UiO-66 is that the MOF’s organic linker, terephthalic acid (TA), is what can be obtained when breaking down plastic.

Structural characterization studies revealed that during the degradation process, UiO-66 undergoes an interesting transformation into another zirconium-based MOF called MIL-140A. This MOF also showed great catalytic activity toward PET degradation.

The title of the paper is “Catalytic Degradation of Polyethylene Terephthalate Using a Phase-Transitional Zirconium-Based Metal-Organic Framework.”

A Northwestern University research team is the first to demonstrate that a material called metal-organic framework (MOF) is a stable and selective catalyst for breaking down polyester-based plastic into its component parts.

It helps a used plastic bottle to be returned to its original components, ready to be made into a new plastic bottle instead of possibly ending up in a landfill. The work was published recently in the journal Angewandte Chemie.

Only three things are needed: plastic, hydrogen and the catalyst. An important bonus is that one of the component parts the plastic is broken down into is terephthalic acid, a chemical used to produce plastic. With the Northwestern method, it isn’t necessary to go all the way back to oil and the expensive and energy-intensive production and separation of xylenes.

“We can do a lot better than starting from scratch when making plastic bottles,” shared Omar Farha, a professor of chemistry in the Weinberg College of Arts and Sciences who is the corresponding author of the study. “Our process is much cleaner.”

The researchers chose a zirconium-based MOF called UiO-66 because it is easy to make, scalable and inexpensive. Yufang Wu, the study’s first author and a visiting graduate student in Omar Farha’s group, used the plastic that was most handy - the plastic water bottles her colleagues in the lab had discarded. She chopped them up, heated the plastic and applied the catalyst.

Catalytic degradation of PET using a phase-transitional zirconium-based MOF.

“The MOF performed even better than we anticipated,” Omar Farha told. “We found the catalyst to be very selective and robust. Neither the color of the plastic bottle or the different plastic the bottle caps were made from affected the efficiency of the catalyst. And the method doesn’t require organic solvents, which is a plus.”

A class of nano-sized materials, MOFs have been widely investigated because of their highly ordered structures. Omar Farha has studied MOFs for more than a decade and previously showed they can be used to destroy toxic nerve agents.

In the current study, MOFs act also in much the same way — breaking an ester bond to degrade polyethylene terephthalate (PET).

“We’ve been using zirconium MOFs to degrade nerve agents for years,” Omar Farha said. “The team then wondered if these MOFs could also degrade plastic even though the reactions and mechanism are different. That curiosity led to our recent findings.”

“This research helps to address long-standing challenges associated with plastic waste and opens up new areas and applications for MOFs,” Farha continued.

MOFs are made of organic molecules and metal ions or clusters which self-assemble to form multidimensional, highly crystalline, porous frameworks. To picture the structure of a MOF, Omar Farha explained, envision a set of Tinkertoys in which the metal ions or clusters are the circular or square nodes and the organic molecules are the rods holding the nodes together.

In addition to being easy to make, scalable and inexpensive, another advantage of UiO-66 is that the MOF’s organic linker, terephthalic acid (TA), is what can be obtained when breaking down plastic.

Structural characterization studies revealed that during the degradation process, UiO-66 undergoes an interesting transformation into another zirconium-based MOF called MIL-140A. This MOF also showed great catalytic activity toward PET degradation.

The title of the paper is “Catalytic Degradation of Polyethylene Terephthalate Using a Phase-Transitional Zirconium-Based Metal-Organic Framework.”